This invention relates to a discharge lamp lighting device, and more particularly to a device for lighting a discharge lamp such as a mercury lamp, a metal halide lamp or the like.
A lighting device for lighting an illuminator such as an illuminator used in a construction site, that for leisure time amusement, that for disaster prevention or the like often uses a synchronous generator as a power supply therefor. Recently, a discharge lamp such as a mercury lamp or the like has been extensively substituted for an incandescent lamp as an illuminator for a construction site or that for leisure. As widely known in the art, a discharge lamp has negative impedance characteristics. Thus, when the discharge lamp starts discharge once, it is decreased in impedance with an increase in discharge current; so that a failure in restriction on the discharge current causes the discharge current to be unlimitedly increased, leading to breakage of the discharge lamp. Thus, lighting of the discharge lamp requires arrangement of a ballast for restricting the discharge current within a suitable range.
A conventional discharge lamp lighting device driven by an internal combustion engine generally includes a synchronous generator driven by the internal combustion engine to induce an AC voltage, an automatic voltage regulator for controlling an output voltage of the generator in a manner to keep it at a set level, a speed control unit for controlling a rotational speed of the internal combustion engine, and a ballast for restricting a current fed from the generator to the discharge lamp, wherein the automatic voltage regulator controls so as to keep an output voltage of the generator at 100V and the speed control unit controls a rotational speed of the internal combustion engine so that an output frequency of the generator is kept at a commercial frequency of 50 Hz or 60 Hz.
The ballast is constituted by a leakage transformer constructed of an iron core having an air space or gap at an intermediate of a magnetic path and primary and secondary coils wound on the core, wherein an output of the generator is applied across the primary coil of the transformer and the discharge lamp is connected across the secondary coil of the transformer.
Typically, the leakage transformer has a phase advancing capacitor for power factor compensation connected across the secondary coil thereof.
The leakage transformer functions to limit a discharge current of the discharge lamp to a predetermined level or below, because output-voltage to output-current characteristics thereof exhibit drooping characteristics which cause an output voltage thereof to be rapidly decreased with an increase in output current thereof to limit the output current to a predetermined level or below.
A discharge lamp lighting device in which a ballast constituted by such a leakage transformer is incorporated is disclosed in Japanese Utility Model Publication No. 27194/1980 (55-27194), Japanese Utility Model Publication No. 17513/1982 (57-17513), Japanese Patent Publication No. 87958/1993 (5-87958) and the like.
The conventional discharge lamp lighting device driven by an internal combustion engine, as described above, requires arrangement of the ballast between the generator and the discharge lamp, leading to an increase in cost thereof.
Also, the ballast acts as an inductive load, resulting in power factor thereof being deteriorated or reduced, so that arrangement of the ballast causes a burden applied to the generator to be disadvantageously increased.
Further, such arrangement of the phase advancing capacitor for power factor compensation at the ballast as described above exhibits a disadvantage of causing flowing of a leading current when the discharge lamp is lighted again after it is turned off once, resulting in an increase in magnetic flux flowing through an armature core of the generator, so that an output voltage of the generator is often excessively increased.
Arrangement of the ballast causes an inrush current increased to a degree about 1.5 times as large as a rated input current to be flowed through an armature coil of the generator during starting operation. Thus, when a rated output current of the generator is conformed to a rated current of the discharge lamp, the generator is rendered overloaded during the starting operation, to thereby cause an increase in voltage regulation, resulting in satisfactory lighting of the discharge lamp being often failed. In order to ensure satisfactory lighting of the discharge lamp, it is required to limit a voltage regulation of the generator to a range of .+-.5 to 6%. This requires to construct the generator into a capacity increased sufficiently to provide a rated current 2 to 3 times as large as a rated current of the discharge lamp, leading to an increase in cost.
In particular, when it is desired to light a plurality of discharge lamps concurrently, the generator and the internal combustion engine for driving the generator are required to exhibit a highly increased capacity, leading to an increase in cost.
It would be considered to render an output of the internal combustion engine and a power capacity of the generator equal to the sum of rated consumption powers of a plurality of discharge lamps and light the discharge lamps in order. Unfortunately, this causes margins in output of the generator to be reduced with an increase in the number of discharge lamps lighted, resulting in the generator falling into an overloaded state at the time of starting of discharge lamps subsequent thereto, leading to a failure in satisfactory starting of the discharge lamps. For example, supposing that four discharge lamps are to be lighted, when three discharge lamps are turned on in order and then the last or fourth discharge lamp is to be turned on, a starting current thereof causes the generator to be overloaded, resulting in a variation in voltage applied to the last discharge lamp being increased, so that lighting of the last discharge lamp is often failed.